G76 screwcutting wizard

I have just converted a Harrison m300 to mach 3 control and the machine works very well . I had a few teething troubles but they are all sorted except one . G76 screwing is all wrong it rapids the wrong way,cuts the wrong way ,the clearance digs in . the first cut is fine as far as pitch is concerned then it does all the other cuts at 3 times pitch. If I use g32 and do it that way its perfect . I can live with this but would like g76 wizard to work . If I do all the g76 by hand it makes no difference . Any ideas? my heads gone numb trying to work it out . Thanks